The present invention relates to a loop back method for a loop type local area network when line failure occurs simultaneously in both of duplex loop transmission lines.
FIGS. 1A.about.1C show a procedure of a loop back operation generally performed when line failure occurs in loop transmission lines.
In the drawings, reference numeral 61 designates a center node unit (hereinbelow referred to as "the CN") having a network supervisory control function, and 62, 63, 64 and 65 some of a plurality of local node units respectively having a connection control function for a subscriber's terminal which are denoted LN(k), LN(k+1), LN(k+2), LN(k+3), respectively. These node units are coupled in a loop shape by duplexed optical transmission lines R [also referred to as a right-direction (clockwise) transmission line] and L [also referred to as a left-direction (counterclockwise) transmission line], the transmission directions of which are opposite to each other.
One (for example, the transmission line R) of the transmission lines R and L is used as a working transmission line while the other (for example, the transmission line L) stands ready for operation as a stand-by transmission line.
In this loop network, all data is formed in accordance with a frame shown in FIG. 2B and transmitted on optical transmission lines. One frame is composed of 64 GS (group slot) and circulated on the transmission lines at intervals of 125 .mu.s (8 kHz).
In FIG. 1A, the right-direction and left-direction loop transmission lines are used as transmission lines in the opposite directions to each other through which frames created by the center node unit 61 are transmitted. Each of the local node units 62, 63, . . . performs a relay operation: reproduces a received signal from the transmission lines, recognizes frame synchronization, and transmits the received data to the succeeding local node.
The data transmission between the node units can be performed only on the working loop or transmission line. In other words, data on the standby loop can merely be relayed at each of the local node units.
Each of FIGS. 1A.about.1C shows a situation where line failure 66 occurs simultaneously on the transmission lines R and L between the LN(k+1) 63 and the LN(k+2) 64.
More specifically, FIG. 1A shows that the CN has detected the failure (interruption of optical input, out of frame synchronization or the like) on both transmission lines and transmitted a simultaneous loop back setting command (hereinbelow denoted "the LBC") to all the LNs through both loop transmission lines.
FIG. 1B shows that all the LNs are performing a loop back operation, wherein each LN maintains a relay operation of the LBC on the loop transmission line through which each LN has received the LBC and returns the LBC on the opposite loop transmission line.
FIG. 1C shows that the loop back operation has been completed, wherein LNs which have received optical signals from both of the loop transmission lines and recognized frame synchronization stop the loop back operation, and the LNs on both sides of the failure location, where frame synchronization has not been established on at least one of the loop transmission lines, still maintain the loop back operation.
FIG. 2A illustrates a block diagram of the configuration of the node unit (CN or LN). As shown in the drawing, there are provided duplex optical-electric conversion units 70R and 70L and duplex loop interface units 71R and 71L corresponding to the right-direction loop transmission line R and the left-direction loop transmission line L.
Between the optical-electric conversion unit 70R and the loop interface unit 71R and between the optical-electric conversion unit 70L and the loop interface unit 71L there is provided a loop switch unit 72 by which a return line for performing a loop back can be formed between both of the loop transmission lines R and L under the control of a node control unit 76, later described in detail.
The loop interface units 71R and 71L are connected to a time-division multiplex unit 74 through a node bus 73.
Incidentally, the loop interface units 71R and 71L in the CN are respectively provided with a frame generating circuit and a time adjusting circuit (not shown in particular) for making corrections for maintaining regular frame circulation.
The time-division multiplex unit 74 is connected to a node control unit 76, a supervisory control means 77 and a terminal interface control means 78 through an internal bus 75.
Although not shown in particular, the node control unit 76 is composed of a microprocessor, a ROM, a RAM, an interrupt control circuit and a variety of interval timers.
The supervisory control means 77 performs a system status monitoring, failure recovery and diagnosis, while the terminal interface means 78 performs a communication control between the terminal and the node unit.
A frame header control unit 79 processes for transmitting and receiving various control command information located in a frame header directed from the CN to the LNs and status information directed from the LN to the CN through the loop interface unit 71R or the loop interface unit 71L, and transmits and receives such information to and from the node control unit 76.
FIG. 2B shows a frame format. One frame is composed of 64 group slots (GS), as shown in the drawing, and is circulated at intervals of 125 .mu.s (8 kHz). The head group slot (GS0) of the frame is a synchronization group slot for the frame synchronization establishment and monitoring control, and the remaining 63 group slots are used for data transmission.
Suppose line failure (loop cut or the like) has occurred simultaneously on both of the right and left-direction loop transmission lines (the loop transmission line R and the loop transmission line L), as shown in FIG. 1A.
The CN detects the line failure as an abnormal frame synchronization and transmits a loop back command to all the LNs to perform a loop back operation.
There have been proposed a variety of loop back control methods. However, since the detection method of the fault location depends on the number of installed LNs on a loop transmission line, a loop back recovery requires a long time in the case where a large number of LNs are installed. For this reason, a concurrent loop back method is adopted which requires a short loop back time independent of the number of LNs. A conventional simultaneous loop back operation will hereinbelow be explained with reference to a timing chart shown in FIG. 3, a processing flow diagram of the CN 61 shown in FIG. 4A and a processing flow diagram of the respective LNs shown in FIG. 4B.
The occurrence of a loop line failure (at a time t0 in FIG. 3) causes the two LNs located closest to the failure location (the LN(k+1) and LN(k+2) in this example) to detect the abnormal frame synchronization of the respective loop transmission lines substantially at the same time (at a time t1 in FIG. 3).
The LN(k+1) and LN(k+2) respectively start a guard timer Tm0 set to the time-out period T0.
As the second closest LNs [the LN(k) and LN(k+3) in this example shown in FIG. 1A] to the failure location, the LN(k) detects a left-direction abnormal frame synchronization on the left-direction loop transmission line L at a time t2, while the LN(k+3) detects a right-direction abnormal frame synchronization on the right-direction transmission line R at a time t3.
Afterward, the abnormal frame synchronization is propagated and detected sequentially by the adjacent LNs. Finally, the CN detects the abnormal frame synchronization on the left-direction loop transmission line at a time t4 and the abnormal frame synchronization on the right-direction loop transmission line at a time t5, and thereby recognizes the abnormal frame synchronization on both of the right-direction and left-direction transmission lines (this state is called "abnormal dual loop frame synchronization). Incidentally, each LN starts the guard timer simultaneously upon detecting the abnormal frame synchronization and checks again the frame synchronization on both of the left-direction and right-direction transmission lines after the time-out period .
Since the respective LNs located on the downstream side viewed from the flow of signals on the right-direction loop transmission line R (for current use of data transmission) detect a right-direction abnormal frame synchronization, the left-direction loop transmission line L (for back up) is switched to operate as the transmission line for current use of data transmission. This switch enables the respective LNs on the downstream side of the failure location to receive a loop back setting command from the CN.
The CN, when detecting the abnormal frame synchronization on both of the transmission lines L and R at the time t5, starts a guard timer Tm1 set to the time-out period T1 and checks again the frame synchronization on both of the transmission lines at a time t6 after the time-out period T1.
If the result of the second check indicates that the dual loop abnormal frame synchronization is still continuing, the CN 61 transmits a loop back setting command to all the LNs. The CN 61 itself also transmits an inputted signal on the working right-direction loop transmission line R to the output side of the standby left-direction loop transmission line L and transmits an inputted signal on the standby left-direction loop transmission line L to the output side of the right-direction loop transmission line R (this operation is called "the dual line loop back"). The CN then starts a guard timer Tm2 set to time-out period T2.
Upon receiving the loop back setting command from the CN, the respective LNs on the upstream side viewed from the flow of signals on the right-direction loop transmission line R (the working transmission line) perform a return from the right-direction loop transmission line R to the left-direction loop transmission line L (hereinbelow called the right-to-left loop back) and start a guard timer Tm3 set to time-out period T3.
The state at this time is as shown in FIG. 1B.
The respective LNs again check the frame synchronization of received data after the time-out period T3. If the result of the second check indicates that both of the right-direction and left-direction transmission lines are both normal as to a given LN, that LN releases its loop back state. If at least one of the right-direction and left-direction loop transmission lines continues the abnormal frame synchronization as to a given LN, the loop back state of that LN is maintained.
On the other hand, CN 61 checks the frame synchronization of received data on both of the transmission lines after the lapse of the time-out period T2 set on the guard timer Tm2, and if the synchronization is normal, then CN 61 releases the dual-line loop back.
Thus, the above-mentioned conventional loop back method is also capable of recovering the left-direction and right-direction loop transmission lines from failure occurring simultaneously on both of the transmission lines.
However, the respective LNs, upon receiving the loop back setting command from the CN, individually set the guard times taking into account a period necessary to recover from a disturbance in the frame synchronization which occurs on the loop transmission lines when they perform the loop back operation. The CN in turn has to set the guard timer so as to cover all time-out periods set on the respective guard timers of all the LNs. Thus, the above-mentioned conventional loop back method encounters a problem in that an extremely long time is required until the loop back operation is finally completed.
The conventional loop back method also implies a problem that the CN does not particularly monitor the operating states of the respective LNs at the time of completing a loop back operation in the whole loop network, so that it is not possible to detect a fault location in a loop back state.
Further, the LNs, if failing to receive a loop back setting command from the CN, can no longer recover to the normal operation.
The aforementioned conventional loop back method further implies a problem in that the node units cannot maintain a loop back state when an extremely short failure such as a momentary interruption of power supply occurs on a transmission line and thereby fall into a loop back failure state.